How to discover Email forwading

Old boss used to have various email accounts forwarded to him for various reasons. New boss wants to know whose mail is being monitored. Without check EVERY account, how can I query to discover ALL the accounts which are being forwarded to a given email account?
Or... how can I query to discover ALL account that are being forwarded to ANY other account?

Last edited by bellzerr; 05-03-2011 at 08:37 AM.
Reason: Added a line.